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MHT-CET Chemistry · Halogen Derivatives of Alkanes

Classification, Nomenclature and Physical Properties

Halides are classed by the carbon that carries the halogen — sp³ (alkyl, allylic, benzylic), sp² (vinylic, aryl) or sp (alkynyl); boiling point rises with the mass of the halogen and the number of halogens, bond strength falls from C–F to C–I, and a carbon with four different groups makes the molecule chiral.

All Halogen Derivatives of Alkanes notesMHT-CET Chemistry strategy

Why this matters

26 PYQs, 1 HARD — the largest page and the most predictable. Eleven ask which formula or name is vinylic, allylic or benzylic (or is NOT); eight ask the highest or lowest boiling point among the halomethanes or the strongest C–X bond; seven ask which halide is chiral or how many chiral carbons it has. Three cards, no calculation.

Concept 1 of 3

Alkyl, Allylic, Benzylic, Vinylic and Aryl Halides

Intuition

Look at the carbon holding the halogen. sp³ and next to nothing special — alkyl. sp³ and NEXT TO a C=C — allylic. sp³ and attached to a benzene ring — benzylic. sp² in a C=C — vinylic. sp² in the ring itself — aryl. sp in a C≡C — alkynyl. One carbon further away, and the label is lost.

Definition

  • Vinylic: X on an sp² carbon of a C=C — CH3-CH=CH-X\text{CH}_3\text{-CH=CH-X}; a haloalkene.
  • Allylic: X on the sp³ carbon ADJACENT to a C=C — CH2=CH-CH2X\text{CH}_2\text{=CH-CH}_2\text{X}, 3-bromopropene, CH3CH=CH-CH2X\text{CH}_3\text{CH=CH-CH}_2\text{X}. NOT CH3CH=CH-CH2CH2X\text{CH}_3\text{CH=CH-CH}_2\text{CH}_2\text{X} (two carbons away).
  • Benzylic: X on the carbon directly bonded to the ring — C6H5CH2X\text{C}_6\text{H}_5\text{CH}_2\text{X} (bromophenylmethane), C6H5CHBrCH3\text{C}_6\text{H}_5\text{CHBrCH}_3, C6H5CBr(CH3)2\text{C}_6\text{H}_5\text{CBr(CH}_3)_2. NOT bromobenzene or 4-bromotoluene (aryl), NOT 1-bromo-2-phenylethane or 1-bromo-2-phenylbutane (X one carbon out).
  • Aryl: X on the ring carbon — chlorobenzene. Alkynyl: X on an sp carbon — a haloalkyne.
  • Primary, secondary, tertiary alkyl halides by how many carbons the C–X carbon carries.

Class by the carbon bearing X

sp3: alkyl / allylic (next to C=C) / benzylic (on ring carbon);sp2: vinylic / aryl;sp: alkynyl\text{sp}^3:\ \text{alkyl / allylic (next to C=C) / benzylic (on ring carbon)};\quad \text{sp}^2:\ \text{vinylic / aryl};\quad \text{sp}:\ \text{alkynyl}

Worked example

Classify: CH2=CH-Cl\text{CH}_2\text{=CH-Cl}, C6H5-CH2CH2Br\text{C}_6\text{H}_5\text{-CH}_2\text{CH}_2\text{Br}, CH3CH=CH-CH2Cl\text{CH}_3\text{CH=CH-CH}_2\text{Cl}.
Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 1Halogen Derivatives of AlkanesEASY
Which among the following is vinylic halide?

[Q91 · 10th May Shift 2 · 2024]

Calling 1-bromo-2-phenylethane benzylic

The bromine carbon is bonded to CH₂, not to the ring. Benzylic needs X on the ring-attached carbon: PhCH₂Br, PhCHBrR, PhCBrR₂.
Drill 10 more on alkyl, allylic, benzylic, vinylic and aryl halides

Concept 2 of 3

Boiling Point and C–X Bond Strength

Intuition

Boiling point follows van der Waals forces, which grow with molecular mass and surface: heavier halogen, more halogens, longer chain — higher. Bond strength runs the other way: the small fluorine overlaps best, so C–F is the strongest and C–I the weakest bond.

Definition

  • Same alkyl: CH3I>CH3Br>CH3Cl>CH3F\text{CH}_3\text{I} > \text{CH}_3\text{Br} > \text{CH}_3\text{Cl} > \text{CH}_3\text{F} in boiling point — fluoromethane lowest, iodomethane highest.
  • More halogens: CHBr3>CH2Br2>CH3Br>CH3Cl\text{CHBr}_3 > \text{CH}_2\text{Br}_2 > \text{CH}_3\text{Br} > \text{CH}_3\text{Cl}.
  • Among isomers, branching LOWERS boiling point (less surface).
  • Bond enthalpy: C–F>C–Cl>C–Br>C–I\text{C–F} > \text{C–Cl} > \text{C–Br} > \text{C–I}; reactivity in substitution is the reverse (iodides fastest).
  • Haloalkanes are denser than water for Br and I; slightly polar, insoluble in water.

Two opposite orders

b.p.: RI>RBr>RCl>RF;bond strength: C–F>C–Cl>C–Br>C–I\text{b.p.: } \text{RI} > \text{RBr} > \text{RCl} > \text{RF};\qquad \text{bond strength: } \text{C–F} > \text{C–Cl} > \text{C–Br} > \text{C–I}

Worked example

Arrange bromoethane, 1-bromopropane, 2-bromopropane and iodoethane by boiling point.
Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 2Halogen Derivatives of AlkanesEASY
Which of the following is correct decreasing order of boiling point of compounds?

[Q96 · 4th May Shift 1 · 2023]

Using bond strength to rank boiling points

The strong C–F bond does not make CH₃F hard to boil — boiling breaks intermolecular forces, not bonds. Fluoromethane has the LOWEST boiling point and the STRONGEST bond.
Drill 7 more on boiling point and c–x bond strength

Concept 3 of 3

Chiral Haloalkanes and Naming From a Structure

Intuition

A halide is chiral when the carbon carrying X — or any other carbon — has four different groups. The exam's recurring quartet is 2-bromopropane (two CH₃), 2-bromo-2-methylbutane (two CH₃), 3-bromopentane (two ethyls) and 2-bromo-3-methylbutane (H, Br, CH₃, isopropyl — chiral). Symmetric diiodides such as 1,4-diiodobutane are inactive.

Definition

  • Chiral: 2-bromo-3-methylbutane, 2-iodo-3-methylbutane, sec-butyl bromide (2-bromobutane), 2-iodopentane, 3-iodohexane.
  • Achiral: 2-bromopropane, 2-bromo-2-methylbutane, 2-iodo-2-methylbutane, 3-bromopentane, 3-iodopentane, n-, iso- and tert-butyl bromide, 1,4-diiodobutane (plane of symmetry).
  • Counting: 2-chloro-3,4-dimethylhexane has THREE chiral carbons (C2, C3, C4); 3,4-dibromohexane two; 1,2- and 1,3-diiodobutane one each; 2,3-diiodobutane two.
  • Naming a drawn halide: longest chain through the double bond, lowest locant to C=C — CH3CH=CH-C(CH3)(Br)-CH3\text{CH}_3\text{CH=CH-C(CH}_3)(\text{Br)-CH}_3 is 4-bromo-4-methylpent-2-ene.

Chirality test

Cabcd, a≠b≠c≠d⇒chiral; two identical groups⇒achiral\text{C}abcd,\ a \neq b \neq c \neq d \Rightarrow \text{chiral; two identical groups} \Rightarrow \text{achiral}

Worked example

How many chiral carbons in 2,3-dibromopentane and in 2-bromo-2-chlorobutane?
Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 3Halogen Derivatives of AlkanesMODERATE
Which of the following isomers of C4_4H9_9Br is a chiral molecule?

[Q96 · 15th May Shift 1 · 2023]

Checking only the carbon that carries the halogen

In 2-chloro-3,4-dimethylhexane C3 and C4 are also stereocentres. Walk every branch-point carbon before you count.
Drill 6 more on chiral haloalkanes and naming from a structure

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (3)

  • Alkyl, Allylic, Benzylic, Vinylic and Aryl Halides

    Class by the carbon bearing X

    sp3: alkyl / allylic (next to C=C) / benzylic (on ring carbon);sp2: vinylic / aryl;sp: alkynyl\text{sp}^3:\ \text{alkyl / allylic (next to C=C) / benzylic (on ring carbon)};\quad \text{sp}^2:\ \text{vinylic / aryl};\quad \text{sp}:\ \text{alkynyl}
  • Boiling Point and C–X Bond Strength

    Two opposite orders

    b.p.: RI>RBr>RCl>RF;bond strength: C–F>C–Cl>C–Br>C–I\text{b.p.: } \text{RI} > \text{RBr} > \text{RCl} > \text{RF};\qquad \text{bond strength: } \text{C–F} > \text{C–Cl} > \text{C–Br} > \text{C–I}
  • Chiral Haloalkanes and Naming From a Structure

    Chirality test

    Cabcd, a≠b≠c≠d⇒chiral; two identical groups⇒achiral\text{C}abcd,\ a \neq b \neq c \neq d \Rightarrow \text{chiral; two identical groups} \Rightarrow \text{achiral}

Watch out for (3)

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